Existing capacitive position sensors directly measure mutual capacitance between relatively movable electrodes to avoid or at least reduce the influence of parasitic capacitance. ASICs (Application-Specific Integrated Circuits) have been the only way to implement such sensors, but only a few applications like digital calipers and levels had markets big enough to write off an ASIC's high development costs.
Recently, several mass-produced microcontrollers with integrated capacitive touch-sensing electronics have become available. These can also be used for position sensing applications other than touch detection. For example, U.S. Pat. No. 7,997,132 to Ross Jr. et al. discloses a capacitive sensor assembly for sensing position or liquid levels through one or more “antenna probes” connected to “an integrated chip normally associated with touch screen displays”.
These microcontrollers sense electrodes' self-capacitance, adequate for keyboard touch detection. However, capacitance to the sensed object is only a fraction of an electrode's self-capacitance, parasitic capacitance, mainly from interconnections, making up the rest. The contribution of this parasitic capacitance, as well as of its drift with contamination such as moisture and condensation, must thus be cancelled. This is usually done by monitoring capacitance in the absence of touch and keeping the touch detection threshold just above it. For position sensing, though, other approaches are needed. An obvious one is to reduce parasitic capacitance, but there are limits: on a printed circuit board for example, most of the parasitic capacitance is through the substrate, with typically 4 to 5 times the dielectric constant of air, and varies strongly with temperature, moisture absorption and condensation.
A well-known and efficient method for removing the effects of parasitic capacitance is by surrounding an electrode with a shield driven by a unity gain buffer having its input tied to the electrode: coupling is virtually eliminated by the nulled shield-to-electrode voltage and by the shielding provided. U.S. Pat. No. 5,166,679 to Vranish et al. discloses a capacitive proximity sensing element backed by a shield driven at the same voltage. U.S. Pat. No. 5,214,388 to Vranish et al. discloses multiple sensing elements backed by a common shield, with circuitry adjusting all sensing element voltages to the shield voltage: this reduces mutual coupling between elements to a negligible level, so that all elements can be sensed simultaneously. A simpler approach is used in the “Electric Field Imaging Devices” integrated circuit family exemplified by the Motorola MC33794 and the Freescale MC33941: multiplexers switch one electrode at a time to the capacitance-sensing circuit and to a unity gain buffer driving the shield output, while the remaining electrodes are grounded. As this does not take care of mutual coupling between electrodes, the data sheets suggest using one coaxial cable per electrode and connecting all shields to the shield output. And unlike microcontrollers, these integrated circuits are not programmable.
In general, unity gain buffers and multiplexers are not integrated in microcontrollers with integrated touch sensing, so they would have to be added to the circuit. Anyway, whether integrated or added externally, unity-gain buffers driving capacitive loads with the bandwidth and slew rate required for the output voltage to follow the input voltage need a much higher supply current than a simple touch-sensing microcontroller without shield driving outputs.
Finally, a drawback of capacitive position sensors in general is the difficulty in monitoring contamination, which can in most cases only be detected when the measured capacitance, or its change, exceeds some threshold. As the sensor is very likely to become unreliable way before the threshold is reached, the warning would come too late.